Radiation is a primary treatment modality for patients with malignant gliomas, and in most patients radiation therapy is clearly beneficial. However, the overall outcome of therapy for these patients is dismal, and most patients with glioblastoma multiforme (GBM) die within a year of diagnosis. The presence of hypoxic cells in brain tumors is a major obstacle for radiation therapy, because these cells are notoriously resistant to radiation-induced damage. Therefore, we propose to devise a gene therapy approach for killing hypoxic brain tumor cells during the course of radiation therapy. The DNA construct to be delivered to the tumor cells contains hypoxia-responsive elements (HREs) in the enhancer region of the promoter and a suicide gene. Under hypoxic conditions, the transcriptional complex hypoxia inducible factor-1 (HIF- 1) builds up in cells and binds to HREs. This, in turn, activates the adjacent promoter and causes expression of the downstream suicide gene that kills the cell. This project has 2 goals. The first is to investigate how several cellular or intratumoral characteristics impact on this gene therapy strategy. The second is to investigate whether the gene therapy enhances the radiation response of the tumor cells. We propose 4 specific aims to accomplish these goals. 1) investigate the relationship between HIF-1 and oxygenation status in brain tumor and normal brain; 2) evaluate suicide genes under low pH and in noncycling brain tumor cells; 3) reveal and investigate any bystander effect (BE) produced by specific suicide genes under hypoxic conditions; 4) determine whether expression of suicide genes in hypoxic and oxic cells enhances their response to radiation.